Hyperthermia has increased the effectiveness of radiation therapy in the treatment of superficial tumors. However, the technical aspects of heating deep seated tumors have not yet been thoroughly solved, although focussed ultrasound and possibly phased array microwave systems have the potential to heat these tumors. An alternative approach to the above methods is to utilize interstitial applicators. This has been relatively simple in cases where the standard treatment has called for an interstitial radiation implant because unfavorable local control rate was expected. The main factors limiting present microwave and RF interstitial heating devices is the small amount of control over the induced temperature distribution alone the length of the applicators and the shallow penetration depth of the applied energy for the techniques relying on thermal conduction (hot sources). These limitations have resulted in subtherapeutic temperatures in many of the tumors treated. In this study we propose to use interstitial ultrasound transducer arrays to improve the temperature distributions and thus clinical responses. First, ultrasound penetrates well in tissue, thus allowing large catheter spacing which can be further extended by utilizing catheter cooling (up to 25 - 30 mm). Second, the transducer elements can be made small enough to permit linear arrays of transducers that still fit into standard brachytherapy catheters to be constructed. By independently driving each of these transducers with separate signals, the temperature distributions along the array can be controlled. Finally, the energy deposition pattern extends to the tip of the applicator and does not depend on the insertion depth or alignment of the catheters. We have shown in preliminary experiments that an ultrasonic transducer array of practical size can be constructed and that enough acoustical power can be produced to induce hyperthermia in vitro in perfused organs and in a spontaneous dog's tumor in vivo. In this proposal we plan to optimize and construct clinical interstitial ultrasonic arrays, develop the driving hardware and software, evaluate the system in animal experiments and then test the feasibility and toxicity of this interstitial hyperthermia technique combined with sequential radiation therapy in a Phase I trial using spontaneous dog tumors. Finally, a Phase I trial with hyperthermia combined with simultaneous radiotherapy will be executed, again in spontaneous animal tumors.